Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2001-11-19
2003-08-05
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Cardiovascular
C600S490000, C600S493000, C600S494000, C600S495000, C600S496000
Reexamination Certificate
active
06602199
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to medical devices, and more particularly, to an apparatus and method for making physiological measurements for diagnostic purposes. The invention has particular application in measuring blood pressure. Specific embodiments of the invention provide automated blood pressure measurement apparatus.
BACKGROUND OF THE INVENTION
Physicians monitor various physiological parameters in their patients and use the results of such monitoring as an important tool to evaluate the patients' health. The monitoring of cardiovascular function is particularly valuable and is performed on a very widespread basis. Accurate measurement of blood pressure and other physiological signals allow for careful diagnosis of medical problems. Monitoring cardiovascular functions, such as blood pressure, can allow a physician to diagnose conditions such as hypertension (increased blood pressure) which may result from processes such as aging or disease.
The heart functions as a pump which moves blood through the circulatory system by a regulated sequence of contractions. The heart ejects blood into the aorta. The blood then flows through the arteries, arterioles, and capillaries to the tissues where the blood delivers oxygen and other nutrients and removes carbon dioxide and other waste products from the tissues. The blood returns to the heart and the lungs where carbon dioxide is expelled from the body and oxygen is again transported into the body. The human body regulates blood pressure throughout the circulatory system to facilitate efficient delivery of blood to the tissues.
Blood pressure does not remain constant but fluctuates during the pumping cycle of the heart. The maximum blood pressure in each cycle is called the systolic blood pressure (“SBP”). SBP occurs as the heart discharges blood into the aorta and the aorta distends to its maximum with the large volume. The minimum blood pressure in each cycle is called the diastolic blood pressure (“DBP”). DBP occurs at the end of the heart's pumping cycle just before the heart begins another contraction. DBP occurs when the aorta has drained of most of the blood from the previous cycle. The mean arterial pressure (“MAP”) is the average of the blood pressure throughout a complete cycle. Mean blood pressure is lower further away from the heart in the circulatory system than it is closer to the heart. Mean blood pressure is also subject to hydrostatic pressure variations. Mean blood pressure tends to be reduced at locations above the heart. Mean blood pressure tends to be greater when measured at locations lower than the heart.
Blood pressure also fluctuates with a wide variety of other factors including activity level, pain, temperature, pharmaceutical agents, stress, and recent smoking or food intake. Studies have shown that the measured blood pressure in a single person can vary considerably over time and in different environments. Results of some of these studies are described in the following references: Watson, R. D. S., et al. “
Variation in cuff blood pressure in untreated outpatients with mild hypertension—implications for initiating antihypertensive treatment” Journal of Hypertension
, Vol. 5, No. 2, pp. 207-211 1987; James, G. D., et al. “
The reproducibility of average ambulatory, home. and clinic pressures” Hypertension
, Vol. 11, No. 6, Part 1, pp. 545-549, 1988; and, White, W. B., et al. “
Average daily blood pressure, not office blood pressure. determines cardiac function in patients with hypertension” Journal of American Medical Association
, Vol. 261, No. 6, pp. 873-877, 1989. Blood pressure is also subject to long-term and more permanent changes due to lifestyle, disease and age.
A complication in taking blood pressure measurements accurately is that many patients suffer from a condition known as “white coat hypertension”. White coat hypertension is a false hypertension that is normally caused by stress and/or anxiety resulting from the presence of a physician or nurse. Studies of white coat hypertension have shown that up to 21% of untreated borderline hypertensive patients had white coat hypertension. (Pickering, T. G., et al. “
How common is white coat hypertension?” Journal of American Medical Association
, Vol. 259, No. 2, pp. 225-228, 1988 and Staessen et al., “
Antihypertensive treatment based on conventional or ambulatory blood pressure measurement” Journal of American Medical Association
, Vol. 278, No. 13, pp. 1065-1072, 1997.)
White coat hypertension may be reduced with familiarity of the patient with the physician, environment, and/or the technology. For example, it has been shown that the blood pressure readings of patients taken by a physician in a clinical environment on two different days two weeks apart tend to drop with time (James et al.,
The reproducibility of average ambulatory, home, and clinical pressures, Hypertension
, Vol. 11, No. 6, Part 1, pp.545-549, 1999).
Various methods are already available for measuring blood pressure. For example, blood pressure may be measured directly in the aorta or other blood vessel. This may be done, for example, by inserting into the blood vessel a probe, such as a needle or catheter which bears, or is attached to, a pressure transducer. The transducer accurately measures the actual pressure of the blood within the blood vessel. Nikolic, U.S. Pat. No. 5,758,652 provides an example of an invasive device capable of making direct blood pressure measurements. The Nikolic device utilizes averaging for smoothing out or removal of respiratory-induced artifacts in an intra-arterial blood pressure signal. Although it is ideal to have directly measured blood pressure values for diagnostic purposes, procedures for directly measuring blood pressure are invasive and are normally restricted to critical care environments such as operating rooms.
Indirect or non-invasive techniques for measuring blood pressure include the traditional method of auscultation in which a blood pressure cuff is inflated to occlude the arteries in the limb (normally the upper arm) and then deflated. During deflation, the physician uses a stethoscope to listen to the Korotkoff sounds (K-sounds) in the blood vessels distal to the blood pressure cuff. The systolic and diastolic pressures are associated with identifiable K-sounds and the cuff pressures at these points are normally measured with a mercury sphygmomanometer. A disadvantage of the auscultation method is that it must be done by a skilled person, such as a physician or nurse.
Another disadvantage of the auscultation method is that it is usually performed by a professional who has limited time. Often only a single measurement is performed. A single measurement may be inaccurate. The American Heart Association recommends that two or more measurements be averaged to produce an accurate determination of blood pressure. Research studies of blood pressure also normally collect multiple readings from patients, and use averaging of the readings in data analysis to limit the effects of fluctuations in pressure and increase accuracy. The measurements should be separated by one to two minutes to allow for release of blood trapped in the veins of the extremity, a wait period that also allows adequate adaptation of the patient to the physician, the environment, and the technology. Making two measurements multiplies the professional time required to obtain a blood pressure measurement.
Barker, U.S. Pat. No. 5,201,320 entitled BLOOD PRESSURE MEASURING DEVICE describes a device designed to address the errors associated with the variability in single blood pressure measurements. The device allows a physician to take two good measurements of blood pressure and to obtain the average of the results automatically. This device requires the physician to use a stethoscope to listen to the K-sounds during automatic deflation of a blood pressure cuff and manually determine the SBP and DBP for each measurement. This device remains subject to the high level of variability and errors associated with white coat hypertension because
Chen Yunquan
Christensen Paul Richard
Gelfer Mark Elliot
Strange Kevin Daryl
Waterman David Sholom
Nasser Robert L.
Oyen Wiggs Green & Mutala
VSM Technology Inc.
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